1. Field of the Invention
The present invention relates to a process for brazing a metal, and particularly, to a process and apparatus for brazing a metal having a high efficient of thermal expansion such as aluminum, copper or the their alloy in the atmosphere by heating by means of a gas burner, wherein a series of operations such as setting, preheating and brazing of a work can be conducted continuously.
2. Description of the Prior Art
There are various proposals made from old times for the automation of a process for brazing a metal and particularly an aluminum article.
For example, a brazing apparatus is disclosed in Japanese Patent Application Laid-open No. 26652/73, in which a work with a brazing filler metal previously applied thereto is placed onto a conveyer and carried through a heating furnace which is divided into six sections, whereby the work is sequentially subjected to drying, preheating, brazing, precooling, annealing and cooling steps during such passage thereof.
An example of an apparatus in which the individual steps are conducted continuously using a turn table is an apparatus described in Japanese Patent Application Laid-open No. 80964/88 previously proposed by the present inventors. In this apparatus, eight work retaining devices are mounted on the turn table, and a work setting step, a work preheating step divided into several stages, a brazing regular heating step, a cooling step and a product removing step are sequentially conducted with turning of the turn table.
In the apparatus described in Japanese Patent Application Laid-open No. 26652/73, the brazing of an aluminum article can be carried out continuously, only by strictly performing a temperature control for sequentially conducting the drying, preheating, brazing, precooling, annealing and cooling steps by using an electric heater and a gas burner for heating and a large-scaled tunnel furnace. This requires great costs of equipment and operation. In a small-scaled production line for making products in a diversified small-quantity production, this apparatus is not commercially practicable from the respect of cost and hence, cannot be employed by any possibility.
The apparatus described in Japanese Patent Application Laid-open No. 80964/88 has been developed by the present inventors for use in a small-scaled production line using a gas burner in the atmosphere. In this apparatus, however, the work is directly heated by the gas burner and hence, a considerable difference in temperature may be produced in the work between a portions near flame from the burner and a portion remoter from the flame, thereby causing a deformation of the work. Therefore, such apparatus can be used practically to a certain extent for brazing of relatively fast works, for example, bonding of a half union with a pipe in a car cooler, but has a disadvantage that it cannot be used practically for works delicate to a thermal shock and works having a shape difficult to heat uniformly, because many fraction defectives are provided. In addition, the prior art process described in such patent also has a disadvantage that a long time is taken for heating the work to a brazing temperature and as a result, the efficiency of production is not enhanced. Further, a variation in work heating time may be produced due to a change in season and a variation in atmospheric temperature in one day, and a corresponding fine control of the temperature is difficult, resulting in defective brazed articles being often produced.